About 40% of world-wide energy consumption can be attributed to the built environment. Therefore, energy conservation in buildings is a key concern for building developers, building inhabitants and governments alike. A significant chunk of building energy usage is related to lighting. For example, in office buildings, many luminaries (e.g. fluorescent tubes) are employed to create sufficient illumination on desk surfaces (a typical value is 500 lux). Daylight dependent dimming is one of the approaches used to conserve energy for lighting. A light sensor is used to measure the ambient light level and the luminary is dimmed down if sufficient light (i.e. daylight from the windows) is already present.
Present day daylight dependent dimming systems typically rely on a sensor built into the luminary itself. That sensor is an integral part of the luminary and controls the light level of the particular luminary depending on the amount of light it senses (more light, more dimming). This is a very simple and relatively cost-effective system, as all control is local to the luminary itself. Installation is also quite straightforward, i.e. it is not different from installing a non-dimming luminary. As a matter of fact, if a luminary is already equipped with a dimmable ballast, the light sensor can be retrofitted into that existing luminary by simply clicking it onto the tube and by making a two-wire connection to the 0-10V control signal input of the ballast. This control signal input is also capable of sourcing a limited supply current to the light sensor.
It should be observed however, that the light sensor doesn't directly measure the amount of tight incident on the working area (i.e. desk surface). Instead it measures light reflected off the working area and/or the floor. In other words, an indirect and consequently inaccurate) measurement is done of the light level on the working area. As part of commissioning, a knob is turned in the sensor to set a set point. This set point will depend on the desired light level on the working area, but also on the reflectivity of that working area and/or the floor. This implies that the set point needs to be changed if the aforementioned reflectivity changes (e.g. new carpet, different desk or more clutter on the desk). Typically this is not done. As a result of the issues mentioned above, these present day daylight dependent dimming systems are not very effective.
Furthermore, it is not possible to adapt the light level to the specific needs of a particular office worker. It should be noted that the light level required is very task and person dependent. For example, an elderly person will have less transparent eye lenses and might need up to four times more light intensity than a younger person.
Systems to directly measure the light level incident on the working area are known in the art. For example, there are desk sensors measuring light level as well as temperature and presence. It is also known to place such desk sensors on the working area (desk surface) and directly measure incident light. The desk sensors use wireless communication to communicate sensor values to the luminary, which adapts its light level accordingly. Furthermore, these desk sensors normally have some User Interface (UI) controls to set a set point, which is also communicated to the luminary.
W0 2010/010491 describes an example of an existing desk sensor. In particular, W0 2010/010491 describes an illumination system that comprises: a light source; a controller controlling the power output of the light source; a light sensor wirelessly communicating with the controller. In a normal mode, the controller controls the light source such that the light level remains substantially constant. In a change mode, the controller controls the light source such that the light level is gradually changed with a predetermined change rate such as to decrease a deviation from a target level. The controller switches from its normal mode to its change mode on the basis of input signals received from the light sensor. The light sensor measures a light level, and decides whether or not to communicate a signal to the controller. The light sensor refrains from transmitting a signal when receiving the signal will not cause the controller to change its control behavior.
A conventional desk sensor still has a number of drawbacks:
It is a relatively costly device.
It needs a power source of its own. One possibility is to recharge a battery with a small PV panel, but this also incurs cost. If the office isn't used for an extended period of time, the battery of the desk sensor may run out.
Commissioning is needed to establish a control relationship between each desk sensor and the most appropriate luminary to control (i.e. the luminary that is closest to the desk). If the desk is moved, this control relationship needs to be adapted. Although relatively simple means for establishing control relationships already exist (see for example the European patent application titled “Method for Establishing Control Relationships, Configuration Device, Networked Device and Computer Program Product”, application Ser. No. 12/182,670.5, filed by NXP B.V. on 31 Aug. 2012) manual action is still needed each time the desk is moved.
On a cluttered desk the light sensor might easily be covered, causing the luminary to produce too much light and hence waste energy.
In view of the above, there exists a need for a lighting control method and system, in particular involving a working area sensor which is ultra-low cost, which has a small firm or, which doesn't require a power source of its own and which is commissioned completely automatically. Furthermore, there is a need to alleviate the “cluttered working area” problem.